Polyolefin polymers can be produced using gas phase polymerization processes. In a typical gas-phase fluidized bed polymerization process, a gaseous stream containing one or more monomers is continuously passed through the fluidized bed under reactive conditions in the presence of a catalyst. The gaseous stream is withdrawn from the fluidized bed and recycled back into the reactor. The recycled gas stream is heated in the reactor by the heat of polymerization. This heat may be removed in another part of the cycle, for example by a cooling system external to the reactor such as a heat exchanger.
The heat generated by the reaction may be removed in order to maintain the temperature of the resin and gaseous stream inside the reactor below the polymer melting point or the catalyst deactivation temperature, or to control polymer properties. Heat removal can also help prevent excessive stickiness of polymer particles that may result in agglomeration. Particle agglomerations may lead to the formation of chunks or sheets of polymer that cannot be removed from the reactor as product. Further, such chunks or sheets may fall onto the reactor distributor plate which may impair fluidization of the bed and may lead to a discontinuity event. Additionally, since the polymerization reaction is exothermic, the amount of polymer produced in a fluidized bed polymerization process is related to the amount of heat that can be withdrawn from the reaction zone.
For a time, it was thought that the temperature of the gaseous stream external to the reactor, otherwise known as the recycle stream temperature, could not be decreased below the dew point of the recycle stream without causing problems such as polymer agglomeration or plugging of the reactor system. The dew point of the recycle stream is that temperature at which liquid condensate first begins to form in the gaseous recycle stream. The dew point can be calculated knowing the gas composition and is thermodynamically defined using an equation of state. However, as described in U.S. Pat. Nos. 4,543,399 and 4,588,790, it was found that a recycle stream can be cooled to a temperature below the dew point in a fluidized bed polymerization process resulting in condensing a portion of the recycle gas stream outside of the reactor. The resulting stream containing entrained liquid can then be returned to the reactor without causing agglomeration or plugging phenomena. The process of purposefully condensing a portion of the recycle stream is known in the industry as “condensed mode” operation. When a recycle stream temperature is lowered to a point below its dew point in condensed mode operation, an increase in polymer production may be possible.
Cooling of the recycle stream to a temperature below the gas dew point temperature produces a two-phase gas/liquid mixture that may have entrained solids contained in both phases. The liquid phase of this two-phase gas/liquid mixture in condensed mode operation is generally entrained in the gas phase of the mixture. Vaporization of the liquid occurs only when heat is added or pressure is reduced. For example, as described in U.S. Pat. Nos. 4,543,399 and 4,588,790, vaporization can occur when the two-phase mixture enters the fluidized bed, with the resin providing the required heat of vaporization. The vaporization thus provides an additional means of extracting heat of reaction from the fluidized bed.
The cooling capacity of the recycle gas can be increased further while at a given reaction temperature and a given temperature of the cooling heat transfer medium. This can be performed by adding non-polymerizing, non-reactive materials to the reactor, which are condensable at the temperatures encountered in the process heat exchanger. Such are collectively known as induced condensing agents (ICAs). Increasing concentrations of ICA in the reactor causes corresponding increases in the dew point temperature of the reactor gas, which promotes higher levels of condensing for higher (heat transfer limited) production rates from the reactor. Suitable ICAs are selected based on their specific heat and boiling point properties. In particular, an ICA is selected such that a relatively high portion of the material is condensed at the cooling water temperatures available in polymer production plants, which are compounds typically having a boiling point of about 20-40° C. ICAs include hexane, isohexane, pentane, isopentane, butane, isobutane, and other hydrocarbon compounds that are similarly non-reactive in the polymerization process.
U.S. Pat. No. 5,352,749, describes limits to the concentrations of condensable gases, whether ICAs, comonomers or combinations thereof, that can be tolerated in the reaction system. Above certain limiting concentrations, the condensable gases can cause a sudden loss of fluidization in the reactor, and a consequent loss in ability to control the temperature in the fluid bed. The upper limits of ICA in the reactor may depend on the type of polymer being produced. For example U.S. Pat. Nos. 5,352,749, 5,405,922, and 5,436,304, characterize this limit by tracking the ratio of fluidized bulk density to settled bulk density. As the concentration of isopentane was increased, they found that the bulk density ratio steadily decreased. When the concentration of isopentane was sufficiently high, corresponding to a bulk density ratio of 0.59, they found that fluidization in the reactor was lost. They therefore determined that this ratio (0.59) was a point of no return, below which the reactor will cease functioning due to loss of fluidization. As described in PCT Publication WO 2005/113615(A2), attempts to operate polymerization reactors with excessive ICA concentrations may cause polymer particles suspended in the fluid bed to become cohesive or “sticky,” and in some cases cause the fluid bed to solidify in the form of a large chunk.
Adding to the complexity of control of stickiness while using ICAs, different polymer products vary widely in their ability to tolerate ICA materials, some having a relatively high tolerance (expressed in partial pressure of the ICA in the reactor), e.g., 50 psia, while other polymers may tolerate as little as 5 psia. In these latter polymers, the heat transfer limited production rates under similar conditions are substantially lower. Polymers which possess a more uniform comonomer composition distribution are known to have a higher tolerance to the partial pressure of the ICA in the reactor. Typical metallocene catalysts are a good example of catalysts that may produce polymers having a more uniform comonomer composition. However, at some point even these metallocene produced polymers reach a limiting ICA concentration that induces stickiness. The limiting ICA concentration depends on several factors in addition to the polymer type, including reactor temperature, comonomer type, and concentration. Further, with the effect of temperature, ICA level, and comonomer levels all affecting on the onset of stickiness, determining the point at which sticking begins to occur has heretofore been difficult.
Two articles by Process Analysis & Automation Limited (PAA), entitled “Agglomeration Detection by Acoustic Emission,” PAA Application note: 2002/111 (2000) and “Acoustic Emission Technology—a New Sensing Technique for Optimising Polyolefin Production” (2000), suggest process control in fluidized bed production of polyolefins may be performed by utilizing acoustic emission sensors located at various positions on the reactor and recycle piping. These publications purport to solve the problem of detecting large polymer agglomerates in a reactor, such as chunks or sheets, rather than detecting stickiness of the resin particles, and provide only one specific example, showing the detection of a chunk of approximately 1.5 meters in diameter within a commercial fluid bed reactor. There is no mention of the detection of polymer stickiness or cohesiveness. In effect, the PAA documents describe the detection of agglomerates after they have been formed in the reactor, rather than detection of resin stickiness that, if left unchecked, could lead to the formation of the agglomerates.
PCT Application Publication Number WO 2003/051929 describes the use of mathematical chaos theory to detect the onset and presence of sheeting in a fluid bed reactor. However, like the PAA articles, the reference does not disclose how to predict when a resin in a reactor is going to become sticky, or any method allowing safe operation of a polymerization reactor near its limit of ultimate cooling capacity for maximum production rates.
WO 2005/113615 and corresponding U.S. Patent Application Publication No. 2005/0267269 describe determination in a laboratory of a critical temperature below which resin in a polymerization reactor cannot become sticky, and use of this predetermined critical temperature to control the reactor.
U.S. patent application Ser. No. 11/227,710 discloses monitoring of resin stickiness during operation of a polymerization reactor by generating a time series of readings of acoustic emissions of the contents of the reactor during steady state operation. Additional acoustic emission measurements are then generated and processed to determine whether they deviate from acoustic emissions indicative of steady state reactor operation. Such deviation is treated as an indication of onset of excessive stickiness of polymer particles in the reactor. Corrective action can be taken (e.g., ICA and/or monomer levels and/or reactor temperature can be adjusted) when the acoustic emission measurements are determined to deviate from those of a steady state reactor. However, this application does not teach the generation of a reference temperature above which resin in a reactor is predicted to become sticky.
Other background references include U.S. Patent Application Publication Nos. 2004/063871, 2005/0267269; 2007/073010, and WO 2005/049663, and WO 2006/009980; and “Model Prediction for Reactor Control,” Ardell et al., Chemical Engineering Progress, American Inst. Of Chem. Eng., US, vol. 79, no. 6, (June 1983).
Even within the constraints of conventional operations, control of reactors is complex, adding further to the difficulty of finding operating conditions that may result in higher production rates. It would be desirable to provide a method of determining a stable operating condition for gas fluidized bed polymerization, especially if operating in condensed mode, to facilitate optimum design of the plant and the determination of desirable process conditions for optimum or maximum production rates in a given plant design. It would also be desirable to have a mechanism in commercial gas-phase reactors to detect the onset of stickiness that is a better or earlier indicator of the onset of stickiness than are conventional techniques (e.g., monitoring the fluidized bulk density as described in U.S. Pat. No. 5,352,749). Such a mechanism would allow the operators to determine when conditions of limiting stickiness are being approached, and enable them to take corrective action before discontinuity events occur, while keeping the reactors at or near conditions of maximum ICA concentration, permitting higher production rates with substantially less risk.